#include<stdio.h>

int main()
{
	int n,i,ans,x;
	while (scanf("%d",&n)==1)
	{
		ans=0;
		for (i=1;i<=2*n+1;i++)
		{
			scanf("%d",&x);
			ans=ans^x;
		}
		printf("%d\n",ans);
	}
	return 0;
}
